In the existing routing solution for mobile communication networks, to support calling of a mobile phone subscriber in roaming state, a voice bear has to be established between the mobile network in the subscriber's home region and the mobile network in the roaming region. Due to the long communication route, the voice quality of roaming service is degraded, and more network resources are consumed; furthermore, the communication cost in roaming state is much higher than that in normal state. Especially, when a subscriber is in international roaming state, all calls to the subscriber are international long-distance calls, i.e., the calls have to be accomplished through a voice bear between the mobile network in the home country and the mobile network in the roaming country, and the roaming subscriber has to pay the charge for international long-distance calls between the home country and the roaming country, even though the calling party is within a stone's throw to the roaming subscriber. For example, when companions in the same tourist party as the roaming subscribers call each other, or a fixed phone subscriber or a mobile subscriber in the roaming region calls the roaming subscriber, a voice bear has to be established in the roaming mode as described above, even though the calling party is very close to the roaming subscriber; in addition, since the voice bear is routed to the roaming subscriber's home country and then detoured, the long-distance international connection results in poor voice quality, and the subscriber has to pay double international long-distance call cost; as shown in FIG. 1, the procedure is as follows:
1. Subscriber B roams in the serving area of Visit Location Register (VLR) B in Public Land Mobile Network (PLMN) A; subscriber A in PLMNA calls subscriber B; the call is switched to the Gateway Mobile-Services Switching Center (GMSC) A;
2. GMSCA routes the call to GMSCB in PLMNB;
3. GMSCB sends a “Send Routing Information” (SRI) message to request for routing information to Home Location Register (HLR) B;
4. HLRB requests for the roaming number to VLRB;
5. VLRB sends a roaming number to HLRB;
6. HLRB returns the roaming number to GMSCB;
7. GMSCB routes the call to GMSCA by determining the roaming number is a number in the country where PLMNA is;
8. GMSCA switches the call to MSCB according to the roaming number. For this problem, a method of support of optimal route (SOR) in the technical specifications of Global System for Mobile Communications (GSM) and Universal Mobile Telecommunication Services (UMTS) proposes that the GMSCA fetches the routing information directly from HLRB, but the method is substantially inapplicable at present in view that the operators tend to protect their subscribers' information. According to the current specifications, the roaming number should generally be obtained from the home network if the route optimization is required; however, the issue of subscriber routing information protection involved in international roaming has not been settled; therefore, route optimization service can't currently be developed and applied on a large scale.